Method and apparatus for controlling a fuel cell system

ABSTRACT

A method of operating a fuel cell includes the step of selectively connecting and disconnecting the fuel cell to at least one electrical load dependent at least in part upon at least one of a fuel cell voltage, a fuel cell current and a fuel cell temperature.

TECHNICAL FIELD

The present invention relates to fuel cells.

BACKGROUND OF THE INVENTION

Automobiles emit hydrocarbons, nitrogen oxides, carbon monoxide andcarbon dioxide as a result of the combustion process. Automobileemissions are said to be a significant contributor to pollution. Inorder to reduce and/or eliminate such emissions automobile manufacturershave attempted to utilize alternative transportation fuels and/oralternative sources of power, such as, for example, fuel cells.Generally, fuel cells generate electricity by electrochemicallycombining across an ion-conducting electrolyte a fuel, such as hydrogen,carbon monoxide, or a hydrocarbon, and an oxidant, such as air oroxygen.

A fuel cell system typically includes a “stack” of individual fuel cellsthat are electrically interconnected in a series configuration. Thus,the number of cells in the stack, i.e., the number of cells connectedtogether in series, determines the voltage that is produced by thestack. Each of the individual fuel cells within the stack produces avoltage that varies dependent at least in part upon the current beingdrawn from that cell and/or the stack. The voltage produced by a typicalsingle cell varies from an open circuit voltage, such as, for example,approximately 1.0 Volts (V) at low or zero current loads to a lowerlimit, such as, for example, approximately 0.7 V, under high currentloads. If the voltage produced by a cell drops below a minimumthreshold, such as, for example, 0.6 V, an undervoltage condition existsthat may result in damage to the cell, such as, for example, celloxidation.

Since the voltage produced by each cell varies dependent at least inpart upon the current load upon the cell, the voltage produced by thestack also varies dependent at least in part upon the current load. Moreparticularly, due to the series interconnection of the cells in thestack, the variation in the voltage produced by the cells is cumulative,i.e., the stack voltage will vary in a manner that reflects the sum ofthe voltage variations of the individual cells within the stack. Thiscumulative effect on the stack voltage can be relatively substantial.For example, the voltage produced by a fuel cell having sixty cells mayvary from approximately sixty volts to approximately forty-two volts.

Most electrical systems are designed to operate with a supply voltagethat falls within a predetermined range. As described above, the voltageproduced by a fuel cell stack may vary substantially. Thus, if a fuelcell system is to be used as a power source for such an electricalsystem the stack voltage must typically be regulated by a voltageregulating device or devices to ensure the stack voltage supplied to theelectrical system remains within the voltage range required by theelectrical system, independent of the voltage produced by the stack. Asthe amount of variation in the voltage produced by the stack increases acorrespondingly greater amount of regulation is required in order toprovide a supply voltage to the electrical system that is within thespecified range. In order to provide adequate regulation of such awidely-varying voltage, voltage regulation or control devices that arerelatively complex, costly, sizeable, and power consuming are required.

Therefore, what is needed in the art is a fuel cell system thatsubstantially reduces damage and/or oxidation to the cells, such as, forexample, due to an under voltage condition.

Furthermore, what is needed in the art is a method and apparatus thatcontrols the output voltage of a fuel cell system while also controllingthe operation of the fuel cell such that the fuel cell operates withimproved efficiency relative to unregulated operation.

Moreover, what is needed in the art is a fuel cell system that generatesa controlled and/or regulated output voltage.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for controllingthe operation of a fuel cell system.

The present invention comprises, in one form thereof, the step ofselectively connecting and disconnecting the fuel cell to at least oneelectrical load dependent at least in part upon at least one of a fuelcell voltage, a fuel cell current and a fuel cell temperature. Theinvention further comprises, in one form thereof, a fuel cell unithaving a fuel cell stack producing a fuel cell voltage and a fuel cellcurrent. A power conditioner electrically connected to the fuel cellunit includes a power switching device. The power switching deviceselectively connects and disconnects the fuel cell voltage to at leastone load dependent at least in part upon an operating temperature of thefuel cell stack, the fuel cell voltage, and the fuel cell current tothereby produce an output voltage.

An advantage of the present invention is that the potential of damageand/or oxidation of the cells, such as, for example, due to an undervoltage condition, is substantially reduced.

Another advantage of the present invention is the output voltage of thefuel cell system is controlled while the operation of the fuel cell isalso controlled such that the fuel cell operates with improvedefficiency relative to unregulated operation.

A further advantage of the present invention is the output voltagegenerated is substantially controlled and/or regulated.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of this invention,and the manner of attaining them, will become apparent and be morecompletely understood by reference to the following description of oneembodiment of the invention when read in conjunction with theaccompanying drawings, wherein:

FIG. 1 is a schematic block diagram of one embodiment of a fuel cellsystem of the present invention; and

FIG. 2 is a schematic diagram of the power converter of FIG. 1.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplification set out hereinillustrates one preferred embodiment of the invention, in one form, andsuch exemplification is not to be construed as limiting the scope of theinvention in any manner.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, and particularly to FIG. 1, there isshown one embodiment of a fuel cell system of the present invention.Fuel cell system 10 includes fuel cell unit 12, power conditioner 14 andfuel cell controller 16.

Fuel cell unit 12 includes a conventional fuel cell stack 18 constructedof a plurality of individual fuel cells (not shown), such as, forexample, solid oxide fuel cells (SOFC), that are electricallyinterconnected in series. Fuel cell unit 12 also includes associatedcomponents, such as, for example, at least one reformer, waste energyrecovery system and conduits interconnecting the components to eachother and with supplies of fuel and/or air, etc (none of which areshown). Fuel cell unit 12 generates a substantially unregulated outputvoltage V_(STACK) and output current I_(STACK).

Power conditioner 14, in general, controls and/or conditions the voltageproduced by fuel cell unit 12 to remain within a desired orpredetermined voltage, and ensures fuel cell unit 12 is operated in arelatively efficient manner in each of the several operating modesthereof. Power conditioner 14 includes power converter circuitry 22,gate drive circuitry 24, control logic 26, and mode controller 28.

Converter circuitry 22 is electrically interconnected with fuel cellunit 12, and receives therefrom V_(STACK) and I_(STACK). Convertercircuitry 22 is also electrically interconnected with external load 32,such as, for example, one or more forty-two Volt loads. Convertercircuitry 22 is further electrically connected, via DC/DC converter 34,to external load 36, such as, for example, one or more twelve Voltloads. Generally, converter circuitry 22 supplies voltage V_(OUT) toexternal load 32 and DC/DC converter 34. A schematic of an exemplaryconverter circuit 22 is shown in FIG. 2.

Converter circuitry 22 includes at least one power switching device 42(FIG. 2), such as, for example, one or more power metal oxidesemiconductor field effect transistors (MOSFETs), integrated gatebipolar transistors (IGBTs) or other suitable power switching devices.Power switching device 42 is electrically connected between fuel cellunit 12 and each of load 32 and DC/DC converter 34. Generally, powerswitching device 42 controls the current flowing from fuel cell unit 12to load 32 and to DC/DC converter 34. Power switching device 42 isoperated in one of three modes dependent at least in part upon thesignal applied to control terminal 42 a, such as, for example, the gate,thereof. In a current blocking mode, such as, for example, anopen-circuit mode, power switching device 42 disallows substantially allcurrent flow from fuel cell unit 12, thereby enabling fuel cell unit 12to operate in a substantially unloaded condition. In another or a firstmode of operation, such as, for example, a pulse-width modulated mode,power switching device 42 is operated in such a manner that the value ofV_(OUT) is maintained within a predetermined voltage range. In yetanother or second mode of operation, such as, for example, a linearmode, power switching device 42 is operated as a series pass throughdevice to thereby maintain the value of V_(OUT) within a predeterminedvoltage range.

Converter circuitry 22 may be configured as a single power switchingdevice 42 interconnected between fuel cell unit 12 and issuing V_(OUT)to external loads. Preferably, however, converter circuitry 22 isconfigured as a conventional linear regulator integrated circuit, suchas, for example, model number 1802 manufactured by Unitrode Corporationof Merrimack, N.H., model number MC78BC30 manufactured by ONSemiconductor Corporation of Phoenix, Ariz., or model number LM1723manufactured from ON Semiconductor Corporation, that integrates onto asingle chip/integrated circuit the unreferenced components, such as thediodes, capacitors, inductors, etc., shown in FIG. 2.

Gate drive circuitry 24, in general, interfaces control logic 26 withpower converter circuitry 22 thereby enabling signals from control logic26 to drive power converter circuitry 22. More particularly, drivecircuitry 24 is electrically connected to and receives control signal 50from control logic circuitry 26, and is electrically connected andissues drive signal 52 to power converter circuitry 22. Drive signal 52is dependent at least in part upon control signal 50. Drive signal 52 iselectrically connected to and received by control terminal 42 a of powerswitching device 42. Thus, the mode in which power switching device 42is operating is dependent at least in part upon drive signal 52. Gatedrive circuitry 24 is configured as a conventional gate drive circuit,such as, for example, model numbers IR2110 or IR2125 manufactured byInternational Rectifier Corporation of El Segundo, Calif.

Control logic 26 is electrically connected to gate drive circuit 24 andto mode controller 28. Control logic 26 issues control signal 50 todrive circuitry 24, and receives converter mode signal 54 from modecontroller 28. Control logic 26 also receives current signal 62 andvoltage error signal 64. Current signal 62 is indicative of the currentbeing supplied by fuel cell unit 12, i.e., I_(STACK), and voltage errorsignal 64 is indicative of the difference between V_(OUT) and areference voltage V_(REF), as determined by, for example, a comparator(not referenced). Dependent at least in part upon converter mode signal54, current signal 62 and voltage error signal 64, control logiccircuitry 26 issues control signal 50 to drive circuitry 24. Controllogic circuitry 26 is configured as a conventional pulse-widthmodulation switching and control logic circuit, such as, for example,model numbers 1802 or 1526A manufactured by Unitrode Corporation ofMerrimack, N.H.

Mode controller 28, as is described more particularly hereinafter,determines the mode in which power conditioner 14 and fuel cell unit 12operate in order to maintain efficient operation and/or increase theefficiency thereof. Mode controller 28 receives and monitors the outputvoltage V_(OUT) of power conditioner 14. Mode controller 28 alsoreceives V_(STACK) and current signal 62, which is indicative ofI_(STACK). Mode controller 28 issues converter mode signal 54 which isindicative of the operational mode that is most efficient given theoperating conditions and parameters of fuel cell unit 12 and powerconditioner 14. Mode controller 28 issues to fuel cell controller 16 acell operational control signal 70, which is indicative of anyadjustments necessary to the output, such as, for example, I_(STACK) andV_(STACK), of fuel cell unit 12 in light of instantaneous operatingconditions and parameters. Mode controller 28 is configured as one ormore logic gates, such as, for example, AND, OR and/or NAND gates.Preferably, mode controller 28 is configured as a microprocessorexecuting mode control software 72.

Fuel cell controller 16, such as, for example, a microprocessor-basedcontrol unit, controls the operation of fuel cell unit 12 dependent atleast in part upon stack signals 74, such as, for example, sensorsignals, indicative of the operating conditions and parameters, such as,for example, the amount of reformate flow, operating temperature, etc,of fuel cell unit 12. Fuel cell controller 16 also receives celloperational control signal 70 from, mode controller 28, receives V_(OUT)and I_(STACK) signal 62. Fuel cell controller 16 controls the operationof fuel cell unit 12 by issuing stack control signals 76 that aredependent at least in part upon stack signals 74, cell operationalcontrol signal 70, V_(OUT) and I_(STACK) signal 62 to adjust theoperational parameters, such as, for example, reformate and air flow, tothereby adjust and/or control the operation of fuel cell unit 12.

In use, fuel cell system 10 supplies output voltage V_(OUT) to loads 32and 36. More particularly, power conditioner 14 in conjunction with fuelcell controller 16 control the operation of fuel cell unit 12 andmaintain V_(OUT) within a predetermined and desired voltage range,thereby rendering fuel cell system 10 suitable for use as a power sourcefor a variety of electrical systems, such as, for example, an electricalsystem of a motor vehicle.

Fuel cell unit 12 has three general modes of operation, i.e., start-up,operating, and cool down modes. During the start-up mode of operation,the fuel cell unit 12 has not reached its intended operationaltemperature. Accordingly, current I_(STACK) is substantially lower thana predetermined or nominal value. The difference between the start-upvalue of I_(STACK) and the nominal value of I_(STACK) is detected bymode controller 28, which, in turn, issues mode signal 54 to controllogic 26. Control logic 26 decodes mode signal 54 and, dependent atleast in part thereon, issues control signal 50 to gate drive circuitry24. Gate drive circuitry 24, dependent at least in part upon controlsignal 50, issues drive signal 52. Drive signal 52 is received by powerconverter circuitry 22 and, more particularly, the control terminal ofpower switching device 42.

Drive signal 52, when fuel cell unit 12 is operating in the start-upmode, places power switching device 42 into a corresponding start-upmode, such as, for example, substantially an open circuit, whereincurrent flow from fuel cell unit 12 to loads 32 and 36 is substantiallydisallowed or precluded. By disallowing current flow from fuel cell unit12, power conditioner 14 enables fuel cell 12 to operate at the opencircuit voltage, thereby reducing the duration of time fuel cell unit 12operates in the start-up mode. Thus, power conditioner 14 expedites fuelcell unit 12 reaching its operating temperature and entering theoperating mode.

When fuel cell unit 12 reaches a predetermined minimum start-up orwarm-up temperature, the value of I_(STACK) has increased and reached apredetermined start-up value. This increase in I_(STACK) is detected andrecognized by mode controller 28 of power conditioner 14 which, inresponse to I_(STACK) exceeding the predetermined threshold, issues anupdated mode control signal 54. Control logic circuitry 26 decodes therevised mode control signal 54 and, in turn, issues control signal 50 togate drive circuitry 24. In response to the revised control signal 50,gate drive circuitry 24 issues drive signal 52 that places powerswitching device 42 in a condition that allows a predetermined andrelatively small amount of current I_(STACK) to flow from fuel cell unit12 through to loads 32 and 36. (i.e., a current-limiting mode). Thisrelatively small flow of I_(STACK) enhances the pre-heating of fuel cellunit 12 and fuel cell stack 18 due to the chemical conversion therein ofreformate and air to electricity, and thereby reduces the amount of timerequired for fuel cell unit 12 to reach its operating or usetemperature.

Once fuel cell unit 12 reaches its operating or use temperature, fuelcell unit 12 exits the start-up mode and enters the operating mode. Thereadiness of fuel cell unit 12 to enter the operating mode is detectedby mode controller 28, through the monitoring of I_(STACK) andV_(STACK), which alters mode signal 54 accordingly. Control logiccircuitry 26 decodes mode signal 54 and issues a corresponding controlsignal 50 to gate drive circuitry 24. Gate drive circuitry 24 issues acorresponding drive signal 52 to power converter 22 thereby causingpower switching device 42 to operate in an appropriate one of the firstor second modes of operation (i.e., the pulse-width modulated mode orthe linear mode), typically the first or PWM mode of operation, asdescribed above.

During shut down of fuel cell system 10, the values of V_(STACK) andI_(STACK) being drawn from fuel cell unit 12 are substantially reducedrelative to the warm-up and operating modes. Mode controller 28 detectsthis shut down condition and issues a corresponding mode signal 54 tocontrol logic 26. Control logic 26 decodes mode signal 54 and, dependentat least in part thereon, issues control signal 50 to gate drivecircuitry 24. Gate drive circuitry 24, in turn, issues drive signal 52.Drive signal 52 is received by power converter circuitry 22 therebycausing power switching device to enter the current blocking oropen-circuit operating mode. With power switching device 42 in thecurrent blocking mode, substantially no current flows from fuel cellunit 12 to loads 32 or 36 thereby ceasing the heat-emitting reactionwithin fuel cell stack 18 and expediting the cooling and/or shut downprocess thereof.

With fuel cell unit 12 in the operating or use mode, i.e., fuel cellunit 12 has reached its operating or use temperature, mode controller 28monitors the difference between V_(STACK) and V_(OUT) in order todetermine the most efficient operating mode of power converter 14 andfuel cell unit 12. As described above, with fuel cell unit 12 in theoperating mode power conditioner 14 operates in a first or pulse-widthmodulated mode when the difference between stack voltage V_(STACK) andV_(OUT) is relatively large, such as, for example, greater thanapproximately 3.0 V. Power switching device 42 is placed into the firstmode of operation through the application of a corresponding drivesignal 52, such as, for example, a pulse-width modulated (PWM) signal.V_(STACK) is controlled by controlling and/or adjusting the duty cycleof the pulse-width modulated drive signal 52. Conversely, powerconditioner 14 operates in a second or linear mode when the differencebetween stack voltage V_(STACK) and V_(OUT) is relatively small, suchas, for example, less than approximately 3.0 V, and when V_(STACK) isless than the desired nominal output voltage, such as, for example,approximately 42 V. Power switching device 42 is placed into the secondmode through a corresponding drive signal 52, such as, for example, avoltage level sufficient to bias power switching device 42 into thelinear region of operation. In the linear region, power switching device42 dissipates a relatively low amount of power and therefore operates ina relatively efficient manner.

It should be particularly noted that mode controller 28 monitorsV_(STACK), I_(STACK) and V_(OUT) to detect start-up, over load and shortcircuit conditions. When fuel cell unit 12 is operating under any one ofthose conditions, mode controller 28 controls I_(STACK) via powerswitching device 42. By controlling the amount of current I_(STACK)being drawn from fuel cell unit 12, mode controller 28 indirectlycontrols the reformate flow through fuel cell unit 12, and therebysubstantially protects fuel cell unit 12 from damage.

While this invention has been described as having a preferred design,the present invention can be further modified within the spirit andscope of this disclosure. This application is therefore intended tocover any variations, uses, or adaptations of the present inventionusing the general principles disclosed herein. Further, this applicationis intended to cover such departures from the present disclosure as comewithin the known or customary practice in the art to which thisinvention pertains and which fall within the limits of the appendedclaims.

1. A method of operating a fuel cell, said fuel cell generating a fuelcell voltage and a fuel cell current and having a fuel cell temperature,said method comprising the steps of: selectively connecting anddisconnecting the fuel cell to at least one electrical load dependent atleast in part upon at least one of said fuel cell voltage, said fuelcell current and said fuel cell temperature, wherein said selectivelyconnecting and disconnecting step comprises the steps of: determiningthat the fuel cell temperature is equal to or greater than an operatingtemperature; comparing the fuel cell voltage to an output voltage from apower switching device; biasing the power switching device into a linearmode of operation when the fuel cell temperature has reached theoperating temperature and when the difference between the output voltageand the fuel cell voltage is less than 3.0 volts; and biasing the powerswitching device with a pulse-width modulated signal when the fuel celltemperature has reached the operating temperature and when thedifference between the output voltage and the fuel cell voltage is atleast 3.0 volts.
 2. The method of claim 1, comprising the further stepof biasing the power switching device into a current-limiting mode whenthe fuel cell temperature has reached a start-up temperature.
 3. Themethod of claim 2, comprising the further step of biasing the powerswitching device into a current-blocking mode when the fuel celltemperature is less than the start-up temperature.
 4. A method ofoperating a fuel cell, said fuel cell generating a fuel cell voltage anda fuel cell current and having a fuel cell temperature, said methodcomprising the steps of: selectively connecting and disconnecting thefuel cell to at least one electrical load dependent at least in partupon at least one of said fuel cell voltage and said fuel cell current,wherein said selectively connecting and disconnecting step comprises thesteps of: determining that the fuel cell temperature is equal to orgreater than an operating temperature; comparing the fuel cell voltageto an output voltage from a power switching device; biasing the powerswitching device into a linear mode of operation when the fuel celltemperature has reached the operating temperature and when thedifference between the output voltage and the fuel cell voltage is lessthan 3.0 volts; and biasing the power switching device with apulse-width modulated signal when the fuel cell temperature has reachedthe operating temperature and when the difference between the outputvoltage and the fuel cell voltage is at least 3.0 volts.
 5. The methodof claim 4, comprising the further step of biasing the power switchingdevice into a current-limiting mode when the fuel cell temperature hasreached a start-up temperature.
 6. The method of claim 5, comprising thefurther step of biasing the power switching device into acurrent-blocking mode when the fuel cell temperature is less than thestart-up temperature.